tweak the is_instance and is_class functions, add more tests

Author: leafo

docs/standard_lib.md

@@ -118,6 +118,10 @@ Returns `true` if `value` is a MoonScript class table, `false` otherwise.
 Returns `false` for instances, `__base` tables, plain tables, and non-table
 values.
 
+Works by checking that the value is a table with a `__base` field set directly
+on it, and that its metatable has a `__call` field (the constructor). Both are
+properties unique to MoonScript class tables.
+
 ```moon
 class MyClass
 
@@ -132,6 +136,10 @@ Returns `true` if `value` is an instance of a MoonScript class, `false`
 otherwise. Returns `false` for class tables, `__base` tables, plain tables, and
 non-table values.
 
+Works by checking that the value is a table whose metatable is a `__base` table
+(identified by having a self-referencing `__index`), and that the value itself
+is not a `__base` table.
+
 ```moon
 class MyClass
 

moon/init.lua

@@ -14,16 +14,20 @@ p = function(o, ...)
     return p(...)
   end
 end
-is_object = function(value) -- deprecated: use is_instance or is_class instead
+is_object = function(value)
   return lua.type(value) == "table" and value.__class
 end
 is_class = function(value)
-  return lua.type(value) == "table" and rawget(value, "__base") ~= nil
+  if lua.type(value) == "table" and rawget(value, "__base") ~= nil then
+    local mt = getmetatable(value)
+    return mt and rawget(mt, "__call") ~= nil
+  end
+  return false
 end
 is_instance = function(value)
   if lua.type(value) == "table" then
     local mt = getmetatable(value)
-    return mt and rawget(mt, "__class") ~= nil
+    return mt and rawget(mt, "__index") == mt and rawget(value, "__index") ~= value
   end
   return false
 end

moon/init.moon

@@ -13,12 +13,15 @@ is_object =  (value) -> -- deprecated: use is_instance or is_class instead
   lua.type(value) == "table" and value.__class
 
 is_class = (value) ->
-  lua.type(value) == "table" and rawget(value, "__base") != nil
+  if lua.type(value) == "table" and rawget(value, "__base") != nil
+    mt = getmetatable value
+    return mt and rawget(mt, "__call") != nil
+  false
 
 is_instance = (value) ->
   if lua.type(value) == "table"
     mt = getmetatable value
-    return mt and rawget(mt, "__class") != nil
+    return mt and rawget(mt, "__index") == mt and rawget(value, "__index") != value
   false
 
 type = (value) -> -- class aware type

moonscript/util.lua

@@ -4,12 +4,16 @@ local unpack = unpack or table.unpack
 local type = type
 local moon = {
   is_class = function(value)
-    return type(value) == "table" and rawget(value, "__base") ~= nil
+    if type(value) == "table" and rawget(value, "__base") ~= nil then
+      local mt = getmetatable(value)
+      return mt and rawget(mt, "__call") ~= nil
+    end
+    return false
   end,
   is_instance = function(value)
     if type(value) == "table" then
       local mt = getmetatable(value)
-      return mt and rawget(mt, "__class") ~= nil
+      return mt and rawget(mt, "__index") == mt and rawget(value, "__index") ~= value
     end
     return false
   end,

moonscript/util.moon

@@ -6,12 +6,15 @@ type = type
 
 moon = {
   is_class: (value) ->
-    type(value) == "table" and rawget(value, "__base") != nil
+    if type(value) == "table" and rawget(value, "__base") != nil
+      mt = getmetatable value
+      return mt and rawget(mt, "__call") != nil
+    false
 
   is_instance: (value) ->
     if type(value) == "table"
       mt = getmetatable value
-      return mt and rawget(mt, "__class") != nil
+      return mt and rawget(mt, "__index") == mt and rawget(value, "__index") != value
     false
 
   is_a: (thing, t) ->

spec/moon_spec.moon

@@ -21,6 +21,12 @@ describe "moon", ->
       class Test
       assert.equal "table", moon.type Test.__base
 
+    it "returns 'table' for __base with inheritance", ->
+      class Parent
+      class Child extends Parent
+      assert.equal "table", moon.type Child.__base
+      assert.equal "table", moon.type Parent.__base
+
     it "returns primitive type for non-tables", ->
       assert.equal "number", moon.type 1
       assert.equal "boolean", moon.type true
@@ -149,6 +155,12 @@ describe "moon", ->
       class Hello
       assert.falsy moon.is_class Hello.__base
 
+    it "returns false for __base with inheritance", ->
+      class Parent
+      class Child extends Parent
+      assert.falsy moon.is_class Child.__base
+      assert.falsy moon.is_class Parent.__base
+
     it "returns false for plain tables and non-tables", ->
       assert.falsy moon.is_class {}
       assert.falsy moon.is_class 123
@@ -163,6 +175,42 @@ describe "moon", ->
       assert.truthy moon.is_class Child
       assert.falsy moon.is_class Child!
 
+  describe "is_instance and is_class with imposter tables", ->
+    it "rejects table with only __base set", ->
+      fake = { __base: {} }
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
+    it "rejects table with __base and non-callable metatable", ->
+      fake = setmetatable { __base: {} }, { __index: {} }
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
+    it "rejects table with self-referencing __index but no metatable", ->
+      fake = {}
+      fake.__index = fake
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
+    it "rejects table with __class set directly", ->
+      fake = { __class: {} }
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
+    it "rejects table whose metatable has __class but not self-referencing __index", ->
+      mt = { __class: {} }
+      fake = setmetatable {}, mt
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
+    it "rejects table with self-referencing __index used as its own metatable", ->
+      -- looks like a __base used as a metatable for itself
+      fake = {}
+      fake.__index = fake
+      setmetatable fake, fake
+      assert.falsy moon.is_class fake
+      assert.falsy moon.is_instance fake
+
   describe "is_instance", ->
     it "returns true for an instance", ->
       class Hello
@@ -176,6 +224,12 @@ describe "moon", ->
       class Hello
       assert.falsy moon.is_instance Hello.__base
 
+    it "returns false for __base with inheritance", ->
+      class Parent
+      class Child extends Parent
+      assert.falsy moon.is_instance Child.__base
+      assert.falsy moon.is_instance Parent.__base
+
     it "returns false for plain tables and non-tables", ->
       assert.falsy moon.is_instance {}
       assert.falsy moon.is_instance 123